ANATOMY AND PHYSIOLOGY 



117 



tubes, or trachea, which receive air through paired segmentally-arranged 

 spiracles. Each spiracle is commonly the mouth of a short tube which 

 opens into a main tracheal trunk (Fig. 170) extending along the side of 

 the body. From the two main trunks branches are sent which divide 

 and subdivide and terminate in extremely delicate tubes, which pene- 

 trate even between muscle fibers; between the ommatidia of the com- 

 pound eyes and possibly enter cells. In 

 most cases each main longitudinal trunk 

 gives off in each segment (Fig. 171) three 

 large branches : (i) an upper, or dorsal, 

 branch which goes to the dorsal muscles; 

 (2) a middle, or visceral, branch, which 

 supplies the alimentary tract and the. 

 reproductive organs; (3) a lower, or 

 ventral, branch, which pertains to the 

 ventral ganglia and muscles. 



In many swiftly flying insects (dragon 

 flies, beetles, moths, flies and bees) there 

 occur tracheal pockets, or air-sacs, which 

 were formerly and erroneously supposed 

 to diminish the weight of the insect, but 

 are now regarded as simply air-reser- 

 voirs. Sacs filled with air lessen the 

 specific gravity of an insect in a fluid 

 medium; but do not increase the buoy- 

 ancy of an insect in the air, unless the 

 contained air is warmer than the sur- 

 rounding air; and in the case of birds, 

 it has been found that the air contained 

 in the bones, though warmer than the 



PIG. 170. Tracheal system of an 

 Surrounding medium, has no appreciable insect, a, antenna; &, brain; I, leg; 



effect on fhVht n ' nerve cord; pt P al P us ! 5 - spiracle; 



n *"%** st, spiracular, or stigmatal, branch; t, 



Types Of Tra.ch.ea.tion. TWO types main tracheal trunk; v, ventral branch; 



,. . ,. ,. -i i f vs, visceral branch. After KOLBE. 



of tracheal system are distinguished for 



convenience: (i) the primary, open, or holopj^u^ti/itype described above, 

 in which the spiracles are functional; (2) the secondary, closed, or apneus-_ 

 jtic^ type, in which the spiracles are either functionless or absent. This 

 type is illustrated in Collembola and such aquatic nymphs -and larvae as 

 breathe either directly through the skin or else by means of gills. The 

 two types are connected by all sorts of intermediate stages. 



